The invention relates generally to limited mode optical fibers used in optical fiber communication systems and in particular to high order mode dispersion compensating optical fibers.
One measure of the performance of optical communication systems is the maximum bandwidth; the highest bit rate supported in the communication system. The bit rate generally refers to the speed in which data is transferred from one place to another. High bit rates permit large quantities of data to be transferred in a short period of time. The bit rate is often limited by physical characteristics of the communication link. For example, optical links typically transfer data through an optical waveguide such as an optical fiber in the form of light pulses. As the pulse of light propagates through the fiber, different wavelengths travel at different velocities. This speed differential of the various wavelengths making up the pulse, referred to as chromatic dispersion, causes a short pulse input to one end of the fiber to emerge from the far end as a broader pulse. This limits the bit rate at which information can be carried through an optical fiber. The effect of chromatic dispersion on the optical signal becomes more critical as the bit rate increases. Chromatic dispersion in an optical fiber is the sum of material dispersion and the waveguide dispersion and is defined as the derivative of the group delay with respect to wavelength divided by the length of the fiber.
Dispersion slope is defined as the rate of change of the total chromatic dispersion of the fiber as the wavelength changes, that is, the derivative of the dispersion with respect to wavelength. It is also know as second order dispersion. Third order dispersion is defined as the rate of change of the dispersion slope with respect to wavelength.
In order to achieve the high performance required by today""s communication systems with their demand for higher bit rates, it is necessary to reduce the effect of chromatic dispersion. Several possible solutions are known to the art, including both active and passive methods of compensating for chromatic dispersion. One typical passive method involves the use of dispersion compensating (DC) fibers. DC fiber has dispersion properties that compensate for the chromatic dispersion inherent in optical communication systems. DC fibers exist that are designed to operate on both the fundamental or lowest order mode (LP01), and on higher order modes.
One desired property of DC fiber is significant negative dispersion. Increasing the magnitude of negative dispersion reduces the length of fiber required to compensate for a large amount of positive dispersion. Another desired property of a DC fiber is low optical signal attenuation. Preferably a DC fiber compensates for chromatic dispersion and dispersion slope, and would be operative over the entire transmission bandwidth. The optical transmission bandwidth typically utilized is known as the xe2x80x9cCxe2x80x9d band, and is conventionally thought of as from 1525 nm-1565 nm. Longer wavelengths are also coming into usage, and are known as the xe2x80x9cLxe2x80x9d band, consisting of the wavelengths from 1565 nm-1610 nm.
Refractive index profiles that support desired higher order modes typically also support undesired higher order modes which can generate unwanted effects. Furthermore, periodic perturbations in the fiber such as periodic bending due to spooling create coupling between the desired high order mode and the undesired high order modes guided in the fiber. Modes having approximately the same propagation constants couple more than modes having significantly different propagation constants. The propagation constant xcex2 is a function of the refractive index n according to the formula xcex2=2xcfx80n/xcex. Thus, in place of the propagation constant xcex2, the effective refractive index or each mode neff may be utilized for each wavelength to determine the strength of coupling between modes.
Typical dispersion compensating fibers are designed as single mode fibers which support only the fundamental or LP01 mode at operating wavelengths. Such fibers are characterized by having relatively low negative dispersion, high optical loss, and small effective area Aeff. These fibers typically have low tolerance for high power, exhibit poor macro-bending loss, and provide limited compensation of dispersion slope. Higher order spatial modes such as the LP02 mode are typically not guided through the fiber.
U.S. Pat. No. 5,361,319 discloses a family of DC fibers that are capable of providing dispersion which is more negative than xe2x88x9220 ps/nmxc2x7km and attenuation of less than 1 dB/km at wavelengths in the 1520 nm to 1565 nm range. Several of the disclosed DC fibers also exhibit a negative dispersion slope in this region. The refractive index profiles are typically designed to have a relatively large difference in refractive index between the central core region and the surrounding cladding when compared to a conventional step index single mode fiber. The fibers also typically exhibit a relatively narrow width for the central core region as compared with conventional step index single mode fibers. The maximum dispersion achievable by these fibers is approximately xe2x88x92100 ps/nmxc2x7km with a dispersion slope of approximately 0.8 to 1.2 ps/nm2xc2x7km. The profile is designed to operate in the LP01 mode, and not to support other higher order modes.
U.S. Pat. No. 5,448,674 discloses an optical DC fiber, containing a power law core refractive index profile, a refractive index xe2x80x9cdepressionxe2x80x9d (i.e., trench) surrounding the core, and a refractive index xe2x80x9crisexe2x80x9d (i.e., ridge) surrounding the trench. The refractive index profile is designed to support the LP01 and LP02 modes, but not the LP11 mode at xcexop, the operating wavelength. Dispersion compensation is accomplished with the optical signal in the LP01 mode. Any optical power which is transferred to the LP02 mode is lost, thereby contributing to a greater system loss.
U.S. Pat. No. 5,802,234 discloses an optical DC fiber with a refractive index profile selected such that the fiber supports the LP01 mode, the LP02 mode, and typically at least one higher order mode. The dispersion is substantially all in the LP02 mode. The total dispersion is more negative than xe2x88x92200 ps/nmxc2x7km over a wide wavelength range. The refractive index profile exhibits an effective mode field diameter which increases with increasing wavelength as the mode energy expands to the refractive index xe2x80x9cringxe2x80x9d area. Such a mode field diameter results in losses in the operating wavelength range of 1525 nm to 1560 nm as the LP02 mode expands into the refractive index xe2x80x9cringxe2x80x9d with increasing wavelength. The DC fiber is designed to be operated in the trough of the dispersion curve, (i.e. close to the cutoff wavelength for the mode). The profile is designed so that the dispersion curve in the operative wavelengths is relatively flat, and thus relatively insensitive to manufacturing variations. The third order dispersion in this region is positive, with the slope increasing, indicative of attenuation losses in the LP02 mode.
A need exists for a dispersion compensating fiber which overcomes these and other drawbacks of the prior art.
The present invention relates in one aspect, to a refractive index profile designed to support higher order spatial modes, and in particular the LP02 spatial mode in an optical waveguide. The waveguide exhibits negative dispersion and negative dispersion slope and negative third order dispersion over the operating wavelength. In one embodiment, the profile is designed with a reduced refractive index depression in the center core region, and is intended to enhance the properties of the dispersion compensating waveguide. In addition, the refractive index profile of the present invention supports the LP02 mode.
A limited mode dispersion compensating optical waveguide according to the present invention includes a center core portion having a center core refractive index. The waveguide also includes an outer core portion surrounding the center core portion and having an outer core refractive index that is greater than the center core refractive index. The waveguide further includes a first cladding portion surrounding the outer core portion and having a first cladding refractive index that is less than the outer core refractive index. The dispersion compensating optical waveguide supports at least one high order spatial mode. In one embodiment, the spatial mode is the LP02 spatial mode.
Another embodiment of the limited mode dispersion compensating optical waveguide of the present invention includes a center core portion having a center core refractive index nCC. In another embodiment, the ratio of the difference between an outer core refractive index nOC and the center core refractive index nCC to the outer core refractive index nOC is greater than about 0.2%, and an outer core portion surrounds the center core portion and the outer core refractive index nOC is greater than the center core refractive index nCC. The optical waveguide further includes a first cladding portion surrounding the outer core portion and having a first cladding refractive index nCL1. The first cladding refractive index nCL1 is less than the outer core refractive index nOC.
Still another embodiment includes a second cladding portion surrounding the first cladding portion and having a second cladding refractive index nCL2 which is greater than the first cladding refractive index nCL1. Yet another embodiment includes a third cladding portion surrounding the second cladding portion and having a third cladding refractive index nCL3 which is less than the second cladding refractive index nCL2. In still another embodiment, the optical waveguide supports at least one high order spatial mode and exhibits negative dispersion and negative dispersion slope over an operative wavelength range. In yet another embodiment, the operative wavelength range is about 1525 nm to about 1565 nm. In yet another embodiment, the operative wavelength range is about 1565 nm to about 1610 nm.
The invention is alternatively embodied in an optical communication system including an optical transmitter for generating an optical signal and an optical transmission fiber optically coupled to the optical transmitter. The system further includes a limited mode dispersion compensating optical waveguide supporting at least one higher order spatial mode, and exhibiting negative dispersion, negative dispersion slope and negative or zero third order dispersion substantially over an operative wavelength range. The limited mode dispersion compensating optical waveguide is optically coupled to the optical transmission fiber. The system also includes a receiver optically coupled to the limited mode dispersion compensating optical waveguide. The receiver generates an output signal in response to the optical signal.